Styli are often used with computer tablets, embedded devices, and mobile computers to allow a user to provide input to the system. These styli often lack the range of mechanical friction behaviors seen with traditional writing instruments such as pens and pencils. This impacts the ability of users to achieve the types of writing and drawing effects they are used to creating with these traditional writing instruments. Moreover, unlike traditional writing instruments on various surfaces (such as paper), these styli have the same tactile feel no matter where on the surface they contact and no matter which what the styli are moving on the surface
Users typically are used to a certain “feel” of their preferred writing instrument and know how to take advantage of that feel. Some users dislike writing on a very hard surface (such as a desktop), so they use blotters to make the pen on the paper feel right. What is really happening here is that the pen sinks into the blotter such that the friction that the user feels goes up as compared to writing on a hard surface. Users definitely have preferences about the amount of friction that they desire when they are writing on a surface.
In addition, many styli lack the tactile feedback of traditional writing instruments on a writing surface that give these instruments their familiar feel. For example, writing with a stylus on a glass surface feels nothing like writing with a pencil on paper. With pen on paper the “feel” is different when the stroke is going in one direction than it is when going in another direction. But pen on glass tends to feel the same in all directions of the stroke. This is because unlike the pencil on paper the friction of the styli on the glass surface is the same no matter what the orientation of the stylus relative to the glass surface.
In addition not having the frictional behavior of traditional writing instruments, computer styli often do not “sound right.” Many studies suggest that sounds play an important part in convincingly making the human brain believe what is being felt and seen. Although some techniques have added sounds to the computer styli these technique are often unconvincing.